1. Field of the Invention
The present invention relates to a power amplifier device. It can be applied generally to low-frequency amplification and particularly to the low-frequency amplifiers of frame deflection circuits in television sets.
In an application of this kind, the power amplifier has the role of activating the vertical deflection of the electron beam of the cathode-ray tube supplying a deflection coil by means of a sawtoothed current. The amplifier must have high gain to deliver a high AC current of about 1 to 3 amperes peak to peak at output.
2. Discussion of the Related Art
It is habitual, for this purpose, to use a power amplifier of the type shown in FIG. 1, with a differential amplifier 1 (e+, e-) followed by an intermediate amplification stage 2 with a compensation capacitor C between its output and its input (to obtain the Miller effect) and a push-pull output amplifier 3 designed to obtain a low output impedance, a maximum sweep of the output signal and high efficiency.
The principle of the push-pull assembly consists in placing two series-connected transistors between the two supply rails, the output terminal being taken at the common connection point between the two transistors, so that only one transistor is conductive depending on the sign of the half-wave of the input signal. This push-pull amplifier can be made by bipolar technology, preferably using a common collector assembly (or follower emitter assembly) for the application referred to herein.
In recent technological developments, in particular so as not to have the problem of a second breakdown of the bipolar transistors, it is preferred to use MOS technology and more particularly an assembly using complementary MOS transistors with common sources are shown in FIG. 2. This assembly requires a command that can be used to improve the output voltage sweep. Thus, rail to rail assemblies have been developed in order to give a maximum voltage sweep at the output that is as close as possible to the voltage difference between the two supply rails or lines obtained by biasing the MOS transistors to the limit of their linear zone under low output impedance.
A well-known general problem related to the use of push-pull output amplifiers is that of connection distortion, also called intersection distortion. This distortion corresponds to the passage of conduction from one transistor to another, for example when the input voltage is no longer positive enough for the upper transistor but is not negative enough to make the lower transistor conductive.
A known approach to resolving this problem consists in making the two transistors simultaneously conductive at rest. This is the class AB or class A operation. FIG. 2 thus shows an example of a class AB assembly used for an amplifier using complementary MOS transistors with common sources. It has two complementary transistors M1 and M2 which are respectively a P channel transistor and an N channel transistor, series-connected between a positive supply rail V+ and a negative supply rail V-(generally the ground). The transistor M1 forms the upper stage while the transistor M2 forms the lower stage, the drains of the two transistors connected in common giving an output terminal OUT of the amplifier. The gate of the transistor M2 forms an input terminal E of the amplifier. A transistor M3 and a transistor M4, respectively a P channel transistor and an N channel transistor, are series-connected between the positive supply rail V+ and the input terminal. The transistor M3 is mounted as a current mirror with the transistor M1. Its gate and its drain are thus connected in common to the gate of the transistor M1.
A source 1 of current I is applied to the drain of an N channel transistor M5. This transistor M5 is mounted as a current mirror with the transistor M4. It thus has its gate and its drain connected in common to the gate of the transistor M4. The source of the transistor M5 is connected to the negative supply rail V-by means of an N channel transistor M6.
This N channel transistor M6 has a geometrical ratio K with respect to the transistor M2 that is the same as the ratio between the transistors M1 and M3. Thus, if the input point IN is open (high impedance), there is a quiescent current I0 equal to K.multidot.I. The transistors M5 and M4 are biased on the same current. They therefore have an identical gate-source voltage Vgs. If ID refers to the drain current, this means that Vgs(M6)=Vgs(M2)=&gt;ID(M6)=K.multidot.ID(M2)=K.multidot.I. Similarly, ID(M3)=ID(M4)=I=&gt;ID(M1)=K.multidot.ID(M3)=K..multidot.I.
This quiescent current I0 is used to reduce the distortion since the output transistor, which is not conductive in the half-wave considered, is made conductive before the other transistor has finished being conductive.
In view of the variations (technological and temperature variations), experience shows that the nominal quiescent current must be about 10 to 15 mA for a peak-to-peak output current of 2 to 3 A.
Thus, the trade-off against the reduction of the connection distortion entails an additional power dissipation that has a non-negligible effect on the efficiency of the power amplifier.